The use of coherent optical receivers becomes a requirement for next generation optical transponders providing high spectral density 100 Gb/s (and beyond) channels over optical WDM (wavelength division multiplexing) networks based on 50 GHz-spacing grids. Indeed, coherent detection provides a good tolerance to linear fiber effects such as chromatic dispersion and polarization mode dispersion. However, another major concern of WDM optical transmission systems is the resilience to non-linear impairments that directly impact the achievable reach and/or the achievable transmission capacity. Such non-linear fiber effects are e.g. self-phase modulation (SPM) and/or cross-phase modulation (XPM).
U.S. Pat. No. 6,600,794B1 describes a MLSE receiver for laser light reflected from an optical disk. Bononi A et al., “Cross-Phase Modulation Induced by OOK Channels on Higher-Rate DQPSK and Coherent QPSK Channels”, Journal of Lightwave Technology, IEEE Service Center, NY, US, vol. PP, no. 18, May 9, 2009, pages 3974-3983, describes a method for estimating the performance of high datarate QPSK channels impaired by XPM.
The non-linear fiber effects distort the amplitude and the phase of the transmitted optical signal and an important element for the compensation of these distortions is the reliable and correct determination of the carrier phase of the optical signal as a phase reference. Consequently, a coherent optical receiver should comprise a carrier phase estimator for the recovery of the carrier phase before decision of the transmitted symbols. As such, an automatic optimization scheme of the carrier phase estimator depending on the non-linear effects incurred by the received optical signal in the optical transmission channel would be beneficial, in order to obtain good estimates of the carrier phase and to thereby mitigate the incurred non-linear impairments and to thereby improve the performance of the optical transmission system. Furthermore, it would be beneficial to provide a measure of the degree of non-linear effects present in an optical transmission channel.